When does a foreign corporation have to file Form 1120-F?

A foreign corporation must file Form 1120-F if it is engaged in a US trade or business at any point in the year (even at a loss), or if it has US-source income on which the tax withheld did not fully satisfy its liability. Many foreign corporations also file a protective 1120-F to preserve the right to deductions and treaty benefits even when they believe they owe no US tax.

What is a 'US trade or business' and why does it matter?

A US trade or business (USTB) is considerable, continuous, and regular activity in the United States — through employees, agents, or a fixed place of business. It's the threshold that determines whether your active business profits are taxed on a net basis in the US. Isolated or passive activity generally does not rise to a USTB.

Does a tax treaty stop the US from taxing my company?

Often, yes. Most US income tax treaties tax business profits only if the foreign company has a 'permanent establishment' (PE) in the US — a higher bar than a US trade or business. Without a PE, treaty-protected business profits are exempt. But you must claim the treaty position on a timely filed Form 1120-F and disclose it on Form 8833; the protection is not automatic.

Can I owe US tax without a US office or employees?

Yes. A dependent agent who habitually concludes contracts on your behalf, or income effectively connected through a US permanent establishment, can create US tax exposure even with no leased office. Conversely, purely digital sales into the US without a US-based presence often do not, by themselves, create a US trade or business under current federal rules — but the analysis is fact-specific.

BACK TO RESOURCES

US Inbound Tax

Foreign Companies Doing Business in the US: Nexus, ECI, Form 1120-F, and Treaty Protection

If your foreign corporation touches the US market, the questions are: do you have a taxable presence, is your income effectively connected, and does a treaty protect you? Getting the answer wrong means back tax, penalties, and a lost treaty claim.

SMAART Tax Team

February 10, 2026

7 min read